Dry milling, in which whole corn is ground or milled to produce products for feed and food uses, is a popular method in the art for processing corn. However, dry milling is typically restricted in its range of products, and it typically is not designed to separate the individual components of corn, except in certain methods wherein the corn is partially degerminated prior to milling.
Another method of processing corn is the dry-grind ethanol process, which conventionally includes dry-grinding whole corn, and adding enzymes and yeast to the cooked corn to produce primarily fuel ethanol. Yet another method is corn wet milling, which produces corn oil, corn gluten feed, corn gluten meal, starch, fiber, and corn steep liquor.
Conventional dry-grind and wet milling have limitations. For example, products of conventional dry-grind ethanol processing typically are limited to ethanol, carbon dioxide, and distillers dried grains with solubles (DDGS). Of these products, DDGS is a low-value animal feed, carbon dioxide has an even lower value and is often merely discharged to the atmosphere, and ethanol, if used for fuel, competes unfavorably with low-priced petroleum products. Thus, the dry-grind ethanol industry presently needs government subsidies and tax waivers, which are likely to be eliminated, for economic survival.
In wet milling, corn is first soaked in water (steeped) for several hours prior to undergoing a series of grinding and separation steps that result in one or more of several products such as corn oil, starch, corn gluten feed, corn gluten meal, fiber, and corn steep liquor. Corn wet milling produces a multitude of high value products, but requires high capital investments in plant and machinery. It also requires large amounts of water, typically 5-9 gallons per bushel of corn, primarily for the purification of starch and for steeping.
The required steeping of corn in conventional wet milling is time-consuming. For typical steeping, corn is soaked in water at about 50° C. and for 22-50 hours. Sulfur dioxide is added, and lactic acid is produced by bacteria. Steeping is done mainly to facilitate a subsequent separation of the germ that contains the oil.
Additionally, the water from the above steeping step (“steep water”) is dilute and has to be evaporated. This requires a significant amount of energy. The evaporated steep water (“corn steep liquor”) is sold as such or added to the corn gluten feed, which is a low-value animal feed.
Further, the corn oil is in the germ of the corn kernel, and germ separation is a complicated process. Water with a specific density is added to the ground corn for transportation to flotation tanks or hydrocyclones, where the oil-bearing germ is separated. The germ then is sent to a series of screens, and the germ is washed with even more water. Next, the germ has to be processed, typically dewatered, usually in germ presses. The pressed germ is typically 50% moisture and 25% oil, with the remainder being germ proteins (albumins and globulins) and other minor components of corn. The germ has to be dried prior to extracting the oil.
If oil extraction is done on site, germ proteins are added to corn gluten feed, which is the lowest value product in a corn wet mill. If the germ is then sold to another processor, the germ proteins are lost. Typical corn wet milling operations recover, at best, less than 85% of the oil of corn.